Max and the Gatekeeper by James Todd Cochrane

A war has been waging for centuries; a magical and technological battle between good and evil. This conflict will affect life everywhere, but not everyone is aware it exists. When twelve-year old Max Rigdon is sent to stay with his grandfather for the summer, he unwillingly enters the fight. The day Max steps off the bus he is marked with an evil curse and immediately becomes the target of evil men and creatures with designs on ruling the universe. Not only does this curse cause Max physical pain but it allows his enemies to track and find him. He soon learns that the reason for this unwanted attention is because his grandfather is the keeper of a gateway; a powerful machine that makes travel to hundreds of strange unknown worlds possible. The enemy will do anything to gain possession of this power in their quest to control all worlds. With the help of his friend Cindy and others, Max must quickly learn the necessary skills to survive if he is to avoid the deadly trap that has been planned for him and his grandfather. A trap that has been decades in the making with Max as the missing piece. If successful it will change life as we know it.